When electrochemically drilling a plurality of holes through a workpiece or workpieces of uniform thickness, the drilling operation is usually stopped when a preselected depth of movement of the drilling cathode has occurred. This is effective when there is a precise location of all the workpieces in exactly the same position or when the exit surface for the cathode is always in exactly the same position with respect to the feed of the cathode. However, differences in dimension of workpieces resulting from permitted tolerances cause the breakthrough surface not always to be at the desired location for the cathode to form the appropriate surface at the exit end of the hole being drilled. This problem is made especially difficult when drilling holes through a hollow body such as a turbine blade or vane as the tolerance buildup is doubled by the varying dimension of the outer surface of the blade on which the blade is supported during the drilling operation and the tolerance in wall thickness of the wall to be drilled.
There have been a number of electrochemical machining methods described and developed in the past. Electrochemical machining involves the use of a tool in the form of a nozzle which is made cathodic and which cooperates with an anodic workpiece through an electrolyte which contacts both the tool and the workpiece. Upon flow of electric current through the electrolyte, material of the workpiece is removed or deplated. These processes have been used to a large extent in drilling small diameter holes in metal which are difficult to drill mechanically.
In the prior art, electrochemical machine operators have relied on manually adjusting the power supply to maintain a constant voltage output from the power supply to drill a series of holes consistent in size from part to part, thereby imposing a constant voltage during the drilling operation. However, changes in the conductivity of the electrolyte caused by temperature, concentration, and contamination fluctuations result in changes in amperage output for a given voltage. Since the amount of material removed in an electrochemical process is directly dependent upon amperage and therefore only indirectly dependent upon voltage, the changes in amperage result in varied hole sizes. Although machine operators adjust voltage to compensate for the changing hole sizes, this generally cannot be done until after the holes are already drilled and measured.
It is an object of the present invention to reduce variation in drilled hole size caused by electrolyte conductivity changes.
It is a further object of the present invention to provide such a method which eliminates the need to control electrolyte temperature, concentration, and contamination as tightly as current methods require, while achieving equal results.
It is yet another object of the present invention to simplify machine operation by eliminating manual voltage control adjustment.